Energy discrepencies depending on frame of reference

In summary, the conversation discusses the concept of frame of reference and how it affects the calculation of energy in a rear-end collision between two cars. It is argued that the energy lost in the collision is frame dependent and that conservation of energy may not apply in non-inertial reference frames.
  • #1
aalnaif
6
0
The other day, I was arguing that a car traveling 70mph rear-ending a car traveling 55mph is the same as a car traveling 15mph rear-ending a stationary car. My argument was that, from the frame of reference of the car that was traveling 55mph, the energy of the other car is k(152), where k = mass/2. But then I thought, from the speedy car's frame of reference, the energy that must be dissipated to slow down to 55mph is k(702-552), which is not equal to k(152).

How come the energy that must be dissipated in the speedy car's frame of reference is different from the energy of the speedy car from the slower car's frame of reference?
 
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  • #2
Energy is frame dependent.

Also, if you are talking about the reference frame of the car, conservation of energy goes out the window because they are not inertial reference frames during the crash. I think the question is ill-posed on that ground. You can ask what it looks like in a moving reference frame not attached to either involved vehicle, but it will be different depending on which one you specify.
 
  • #3
Welcome to PF!

Hi aalnaif! Welcome to PF! :wink:
aalnaif said:
… But then I thought, from the speedy car's frame of reference, the energy that must be dissipated to slow down to 55mph…

But it won't slow down to 55mph … why should it?

If you take into account the actual final speeds of the two cars, you'll find that the energy lost is the same in both frames. :smile:
 

1. What is the concept of "energy discrepancies depending on frame of reference"?

The concept of "energy discrepancies depending on frame of reference" refers to the fact that the amount of energy measured in a physical system can vary depending on the observer's frame of reference. This is due to the principles of relativity, which state that different observers can have different perceptions of the same event depending on their relative motion.

2. Why do energy discrepancies occur in different frames of reference?

Energy discrepancies occur in different frames of reference because of the relationship between energy and mass, as described by Einstein's famous equation E=mc^2. This means that the amount of energy measured in a system will depend on the relative motion of the observer, as well as the mass of the objects involved.

3. How do scientists account for energy discrepancies in different frames of reference?

Scientists account for energy discrepancies in different frames of reference by using the principles of relativity and the laws of physics to make calculations and predictions about the behavior of energy in different frames of reference. They also use experimental data to validate these calculations and theories.

4. Can energy be created or destroyed in different frames of reference?

No, energy cannot be created or destroyed in different frames of reference. This is because of the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or transformed from one form to another. The amount of energy measured may vary depending on the frame of reference, but the total amount of energy in the system will remain constant.

5. How do energy discrepancies in different frames of reference impact scientific research and applications?

Energy discrepancies in different frames of reference have a significant impact on scientific research and applications, particularly in fields such as astrophysics and particle physics. Understanding and accounting for these discrepancies is crucial for making accurate predictions and calculations, and for developing new technologies and theories that rely on precise measurements of energy.

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